MockObject should provide a way to test methods like __str__

[ivancrneto]

class Foo:
    def __str__(self):
        print "Foo!"

class TestSuite(unittest.TestCase):
    def setUp(self):
        self.m = mox.Mox()
    def tearDown(self):
        self.m.UnsetStubs()

    def test_Foo_str(self):
        f = self.m.CreateMock(Foo)
        self.m.ReplayAll()
        res = str(f)  # res receives '<MockAnything instance at xx>'
        self.m.VerifyAll()

In most situations, MockObject.__str__ is useful but in some, it prevents from
testing a class's str method. Instead of receiving '<MockAnything...>' in
the example above, there should be a way to make it receive "Foo!"